The present invention relates to an X-ray imaging system and more particularly to a micro focus X-ray imaging system using a transmission-type target which is suited to the high resolution X-raying test.
The focal spot size of the X-ray source may be cited as a factor for controlling the detecting resolution in the X-raying test. Therefore, in the X-raying test requiring high resolution, a micro focus X-ray source is used.
Targets used for the micro focus X-ray source are broadly divided into the transmission type which uses X-rays which transmit a target and the reflection type which uses X-rays which are emitted sideways. From a viewpoint of the resolution, the transmission type is superior because the spread of electron beams in a target can be minimized. An X-ray source using a transmission type target is indicated, for example, in Japanese Patent Application Laid-Open 2-138856, Japanese Patent Application Laid-Open 3-274500, and Japanese Patent Application Laid-Open 4-144045. X-rays of a minute focal spot size which are obtained by irradiating a focusing electron beam to a transmission type film target are irradiated to a sample, and an image is picked up by enlarging and projecting the transmitted X-rays geometrically, and the minute internal structure of the sample is observed.
To improve the resolution it is necessary to reduce the spot diameter of a focusing electron beam and make the film thickness of the transmission type target thinner. However, when the film thickness of the transmission type target is made thinner or the acceleration voltage for the focusing electron beam is increased, the intensity of X-rays generated is oriented and a bright spot is generated at the center of the transmitted X-ray image. As a result, the image quality is extremely lowered.
When the spot diameter of a focussed electron beam is reduced, the current density of a focussed electron beam which is irradiated to the target is increased and the target damage is increased.